Because there is nothing in the Pokemon universe which contains
A) Quality, or the type of quality Nintendo is known for.
B) Nostalgia for things I remember playing as a kid.
C) The sort of kickass character a 24 year old adult such as myself can find appealing on any level whatsoever.
Basically, the basic reason is that Pokemon serves to cheapen the entire stable of Nintendo characters, moreso than Mario Party or Starfox Assault ever could.
The entire series offers no hope of ever changing to anything a detractor could love, and more specifically, ever producing a single console iteration that is even average.